PHITS-Based Monte Carlo Simulation for Radiation Protection Calculation in Small Modular Reactors

Small Modular Reactors (SMRs) are emerging as a transformative low-carbon energy technology, offering compact design, enhanced inherent safety, and flexible deployment for electricity generation, industrial process heat, and seawater desalination. Their smaller cores and novel configurations, however, introduce radiation protection challenges that calculation methods developed for large conventional reactors do not fully address. Compact geometries lead to higher neutron leakage at reactor boundaries, while new coolant types and multi-unit siting arrangements complicate the prediction of radiation doses to workers, the public, and the environment. This research develops and validates a method for calculating the radiation protection of SMRs using PHITS (Particle and Heavy Ion Transport code System), a Monte Carlo particle transport simulation tool developed by the Japan Atomic Energy Agency and adopted here as the primary research tool. The method is being validated against the Akademik Lomonosov floating nuclear power plant, the only SMR-class reactor with several years of real operating experience, and is designed for extension to other contemporary SMR designs. Using PHITS, the approach simulates the generation and transport of neutron and gamma radiation from the reactor core, converts the results into radiation dose estimates at key locations, and evaluates shielding configurations against internationally recognised safety limits, following the As Low As Reasonably Achievable (ALARA) principle. By producing a reproducible and transparent PHITS-based calculation method, this research supports safer and more cost-effective shielding design, informs regulatory decision-making, and addresses a gap identified by international nuclear safety bodies as a priority for the safe deployment of next-generation reactors. The findings are directly relevant to engineers, regulators, and policymakers advancing new nuclear technologies worldwide.